Magnetic particle core manufacturing process

ABSTRACT

MAGNETIC PARTICLE CORES SUITABLE FOR PHASE SHIFTING APPLICATIONS HAVE A PERMEABILITY SUBSTANTIALLY ABOVE 275 AND CORE LOSSES BELOW 0.24 OHM PER HENRY PER UNIT OF INDUCTANCE OF 1800 HERTZ ARE PRODUCED BY A NOVEL COMBINATION OF STEPS INCLUDNG ONE OR MORE OF THE FOLLOWING: REDUCTION OF PRE-POWDER ANNEAL ADDITIVES, SELECTIVE DISTRIBUTION OF MAGNETIC PARTICLE SIZES, ADDING FLEXIBILITY TO THE ELECTRICAL INSULATION APPLIED TO THE MAGNETIC PARTICLES, ELEVATED PRESSURE COMPACTING, SURFACE ETCHING OF THE PRESSURE COMPACTED CORE AFTER HYDROGEN ANNEAL, AND AN OXIDATION HEAT TREATMENT FOLLOWING SURFACE ETCHING. SURFACE ETCHING AND OXIDATION COMBINE TO INCREASE THE BREAKSTRENGTH AND IMPROVE OTHER PROPERTIES OF VARIOUS PERMEABILITY CORES, FOR EXAMPLE, FROM 125 THROUGH 300 PERMEABILITY.

United States Patent 70 3,666,571 MAGNETIC PARTICLE CORE MANUFACTURINGPROCESS Alfred M. Laing, Butler, Pa., assignor to Spang Industries, Inc.No Drawing. Original application Mar. 21, 1968, Ser. No. 714,805, nowPatent No. 3,607,462, dated Sept. 21, 1971. Divided and this applicationOct. 13, 1970, Ser.

Int. Cl. H01f l /24 US. Cl. 148-104 6 Claims ABSTRACT OF THE DISCLOSUREMagnetic particle cores suitable for phase Shifting applications have apermeability substantially above 275 and core losses below 0.24 ohm perhenry per unit of inductance at 1800 hertz are produced by a novelcombination of steps including one or more of the following: reductionof pre-powder anneal additives, selective distribution of magneticparticle sizes, adding flexibility to the electrical insulation appliedto the magnetic particles, elevated pressure compacting, surface etchingof the pres sure compacted core after hydrogen anneal, and an oxidationheat treatment following surface etching. Surface etching and oxidationcombine to increase the breakstrength and improve other properties ofvarious permeability cores, for example, from 125 through 300permeability.

This is a divisional application of application Ser. No. 714,805, filedMar. 21, 1968, and now Pat. No. 3,607,462.

This invention relates generally to preparation of magnetic particlesfor compacting into magnetic components and, more particularly, to highpermeability particle cores exhibiting low core losses in the audiofrequency and related frequency ranges, and to methods of manufacturingsuch cores.

A basic process for manufacture of particle cores is disclosed in theUS. patent to Bandur #2,l05,070. The process steps include thepreparation of magnetic particles, electrically insulating theparticles, compacting into a magnetic core, and annealing the core. Thisbasic process has been capable of producing molybdenumcontainingPermalloy particle cores having a permeability of about 125 and havingcore losses acceptable for audiofrequency applications.

Attemps to improve on this process have been numerous and continuoussince its origin. The problem is to increase the permeability withoutdestroying usefulness of the core due to an increase in core losses.Successful improvements of core properties have generally resulted fromtreatment of the compacted core, for example by boiling, solutiontreatment, and re-anneal of the compacted core as disclosed in the US.patents to Harendza-Harinxma #2,977,263, issued Mar. 28, 1961,#3,014,825, issued Dec. 26, 1961, and #3,132,952, issued May 12, 1964.

The primary objective of particle core improvement endeavor is toincrease the permeability of such cores while maintaining core losseswithin acceptable limits. Particle cores are used in electrical circuitsoperating at voice frequencies and related frequencies up to about20,000 cycles per second where low core losses are a majorconsideration. Also these cores find certain applications at much higherfrequencies which further accentuate the importance of low core losses.In the audio frequency and related frequency ranges, componentsapproaching theoretical perfect reactance are desirable in order toobtain high quality performance. In filters, for example, shortercut-01f, better defined resonance, and higher attenuation ratios arerealized with high quality inductors. Quality in inductors can beassessed from the Q" factor. The Q of inductors is defined as a ratio ofreactance to resistance:

: Rno-lxc where f=frequency in cycles per second L=inductance inhenries, and

R =wire resistance in ohms R =resistance (ohms) due to losses of thecore, including eddy current losses, hysteresis losses, and residuallosses.

Provided core losses can be kept within industry specifications, thereare a number of reasons why increasing the permeability is important,for example, savings on core material, savings on wire, and improvedcircuit design because of smaller components. As is well known in theart, efforts to date to improve on metallic particle cores have resultedin commercially available cores with acceptable core losses having anupper limit of 200, and slightly higher, permeability. Speciallaboratory techniques may yield acceptable cores of about 240permeability however, it has not been possible to produce a particlecore having a permeability above 275 with core losses acceptable in theindustry for the uses considered above.

The present invention teaches novel procedures for preparation ofmagnetic powders for magnetic components and, specifically, preparationof molybdenum-containing Permalloy powders for producing magnetic coresof higher permeability, lower core losses, increased mechanicalbreakstrength, more linear temperature characteristic, and bettermoisture resistance. These teachings enable commercial production ofmolybdenum-containing Permalloy powder cores having a permeabilitysubstantially above 275 and having core losses within industryspecifications.

Core losses are reflected in the windings of a core as resistance lossesand constitute loss of energy in an inductor. Core losses, as used inthe core manufacturing art, include eddy current losses which varydirectly with the square of the frequency, hysteresis losses which varydirectly with the flux density and the frequency, and residual losseswhich vary directly with the frequency.

These core losses may be expressed by the V. E. Legg formula covered inLeggs paper entitled, Magnetic Measurements at Low Flux Densities Usingthe Alternating Current Bridge from the Bell System Technical Journal ofJanuary 1936. This formula is as follows:

Residual loss Hysteresis loss I Eddy current loss I I Total loss factorTo meet industry standards, cores must have a total core loss no higherthan 0.240 unit (ohms per unit permeability per unit inductance) at 1800hertz (cycles per second), with a core loss of 0.200 unit at 1800 hertzbeing the accepted average.

To meet these standards and improve magnetic and mechanical propertiesas well, several novel steps are combined in the present invention.These steps will be described in relation to production of novel highpermeability molybdenum-containing Permalloy powder cores withacceptable core losses. Previously, the highest permeability core, ofthe type discussed, available commercially was a 200 perm core. Thepresent teachings make the 300 perm core commercially available. Ofspecial significance is the fact that these 300 perm cores have lossesacceptable within industry standards for audiofrequency uses, i.e. below0.240 unit.

Molybdenum-containing Permalloy is an alloy consisting essentially ofnickel, iron and molybdenum. In practice, the standard alloy containsabout 2% molybdenum, about 81% nickel and the balance iron. However,alloy constituents can vary within the following percentages withoutserious detriment to properties:

Percent Molybdenum 1.6-4.0 Nickel 78.0-83.0 Iron Balance Methods forpulverizing the alloy are known, e.g. the patent to Beath et al.#1,669,649. 'In practice the metallic constituents of the alloy aremelted together and additives which embrittle the alloy are made in themolten state. This treatment facilitates a fine crystalline structure inthe solidified alloy that enables reduction to a fine powder byconventional rolling, grinding, and pulverizing techniques.

In departing from prior practice, the invention teaches selection of avariety of particle sizes to obtain an optimum packing factor, i.e. anoptimum density of magnetic powder and optimum space for electricalinsulation (electrically equivalent to air space) in the compactedproduct.

A typical particle size distribution to produce 300 perm cores inaccordance with the process of the present invention is:

*l20 to +230 covers particle sizes which will pass through a 120 meshscreetn but will not pass through a 230 mesh screen using normal sieveprac ice.

The important concept discovered here is that the optimum packing factorto obtain high permeability and acceptable core losses is not obtainedby use of single particle size screening but rather by selectivescreening and distribution of particle sizes. The highpermeabilityacceptable core loss product of the invention can beobtained by selecting particles with about one part by weight having anaverage particle size of 90 microns, about three parts by weight havingan average particle size of 65 microns, and about six parts by weight inwhich the average particle size is no greater than about 37 microns.These proportions can be broadened to emphasize certain core propertiesand changes in the electrical insulation also permit variation in theseproportions. However, a larger overall-average particle size tends toincrease both permeability and core losses while smaller overallaverageparticle size tends to decrease both permeability and core losses.

After pulverizing and sieving, the powder is annealed to relieve thestrains induced during brittle practice, that is during the productionof the powder. To prevent welding of the particles during this anneal,additions of nonagglomerating material must be blended with the powder.Such material must remain non-reactive or inert at powder annealingtemperatures. In the prior art, preanneal additions constituted about0.3% to 1.0% by Weight of the metal powder. An important discovery ofthe preent invention relates to better use of the limited amount ofdistributed non-magnetic gap available in producing the higherpermeability product of the present invention; that is, this space canbe better used to provide more effective electrical insulation of theparticles rather than being occupied by pre-anneal additives-When propersteps are followed. By the procedures of the present invention, thepre-anneal additive is drastically reduced to about .02% to about .05 byweight of the metallic powder; preferably such additives are held belowabout 03% by weight. Typically ceramic clays such as talc or kaolin areadded to prevent agglomeration; a preferred pre-anneal additive ispowdered kaolin.

The subsequent powder anneal is held to a temperature of about 1250 F.for about 1 /2 hours in a non-oxidizing atmosphere, e.g. an atmospherecontaining free hydrogen. Temperatures significantly higher than about1250 F. are avoided in order to eliminate agglomeration of the metalparticles. With the present invention it is possible to avoid the 1400F. to 1600 F. powder anneals of the prior art without sacrificingelectrical properties. In fact, a higher permeability-low core lossproduct is obtained.

During the powder anneal the water of crystallization of the kaolin,which constitutes about 13% by weight of this pro-anneal additive, isdriven off. The kaolin should be in the uncalcined condition before thepowder anneal since it has been found that calcined kaolin is not aseffective as standard kaolin in preventing agglomeration of the metalparticles.

After annealing, the work product is sieved through a 50 to mesh screento remove any lumps which may be formed; however, this does not changethe analysis of the metal powder sizes which have been selected toproduce the desired packing factor. This sieving merely breaks up looselumping which may occur.

The magnetic powder is then electrically insulated utilizing a slurryincluding, by present practice, a silicate, an inert metallic oxide, anda ceramic clay. For example, a solution containing about 67 grams ofsodium silicate, about 100 grams of milk of magnesia, and about 6000 cc.of deionized Water is prepared to insulate about 50 pounds of powder.The electrical insulation is applied in a plurality of coats with thefirst coat ordinarily not including a ceramic clay additive in order toutilize the preanneal non-agglomerating additive present with the metalpowder. Subsequent coats after the first coating, utilize about 1400 cc.of the above solution with about 22 grams of powdered kaolin added. Inpreferred practice the electrical insulation is applied in four separatecoats with intermediate dryings of the coatings being carried out attemperatures up to about 315 F. The total electrical insulation, dryweight, is less than about 0.4% by weight of the metal powder weight.

Highly beneficial results are obtained by the addition of a plasticizercoating during the insulation process. Plasticizer as used herein,refers to a material for imparting flexibility to the electricalinsulation or a major ingredient of the electrical insulation, e.g. themetallic silicate in the disclosed insulation. As taught by the presentinvention, the plasticizer must maintain this capability of impartingflexibility to the electrical insulation during processing steps up toand including the compacting step.

Preferably the plasticizer is added as a final coating to the insulatedparticles; however plasticizers exist which require intermediate coatingfor best results. Suitable plasticizers of the latter category includestarches, sugars, or glycerin and a wetting agent. A plasticizersuitable for adding as a final coating is ammonium lignosulfonate in aliquid carrier.

The purpose of the plasticizer is to impart flexibility to theelectrical insulation and permit higher than usual pressures duringcompacting of the particles while avoiding mechanical cracking of theelectrical insulation. In accordance with the teachings of theinvention, the plasticizer should maintain its capability of impartingflexibility during the temperatures encountered in applying electricalinsulation and those encountered in compacting. Preferably theplasticizer should be driven off during the high temperature core annealor, at least, not impair the insulation or leave a reaction producthaving reduced electrical insulation properties.

After the insulation process, including the use of a plasticizer, theinsulated powder is sieved through a 50 to 100 mesh screen to removelumps and chips of insulation. This sieving is carried out withoutchanging the basic magnetic particle sieve analysis.

The insulated powder is pressed into cores at a pressure which issignificantly higher than that previously specified formolybdenum-containing Permalloy powder cores. The compacting pressuretaught by the present invention for the production of higherpermeability cores is ordinarily in the range of about 135 to 150 tonsper square inch and preferably is about 140 tons per square inch. Theplasticizer makes the insulation more flexible and reduces compactingfriction.

Without a plasticizer, the core losses increase considerably at thehigher compacting pressures taught. A portion of the decrease in corelosses available with the present invention can be traced to thedecrease in surface welding stemming from the decrease in compactingfriction. The reduction in core losses also stems from decreasingmechanical breakage of the electrical insulation between particles whichapparently existed in the prior art practice. Further, the plasticizerreduces the amount of lubricant needed in pressing.

However there are limits to the amount of plasticizer which can besafely used since mechanical breakstrength of the core decreases rapidlyabove certain low level percentages. When ammonium lignosulfonate isused with an insulation containing sodium silicate, the amount of dryplasticizer should be about .06% by weight of the metal powder weight.

The invention includes discovery of a step to maintain desiredmechanical breakstrength of the finished product. The co-action of thisstep, which will be described later, offsets any weakening efiect on thecores caused by the plasticizer so that cores with mechanicalbreakstrength equivalent to prior art cores, without plasticizer, cannow be made notwithstanding the use of a plasticizer.

After pressing, the cores are annealed between about 1000 F. and about1500 F., preferably about 1250 F. for approximately 40 minutes in anonoxidizing atmosphere, for example, an atmosphere containing purehydrogen. The cores are quenched in water after removal from theannealing furnace.

Practice of the process of the invention described thus far consistentlyproduces cores within the normal tolerance range for 300 permeabilitcores; however during production runs core losses can vary beyondaccepted standards. To commercially produce cores with lossesconsistently within industry specifications, that is below .240 unit,the invention teaches use of a surface etching step. By surface etchingis meant removal of the skin effect resulting from present-daycompacting techniques used in commercial production ofmolybdenum-Permalloy cores.

The cores are surface etched subsequent to the hydrogen anneal whichfollows pressing. The core should not be surface etched prior to thisanneal. In general, chemical etching is preferred in order to avoidadding any mechanical strains to the particles. A typical etchingpractice utilizes a 50% nitric acid solution with an etching time ofseconds, plus or minus 5 seconds with temperature maintained at 80 F.:5F. An alternate etching 6 procedure is approximately 3 minutes in 40Baum nitric acid with temperature maintained at F.i5 F.

Surface etching can cause a slight decrease in permeability but thisdecrease is limited to about .5 to about 5% of the core permeability.Typically, a 302.1 permeability core may be reduced to 300.6permeability and a 323.3 permeability core may be reduced to 317.3.However, core losses decrease at a much greater rate than permeability;decreases in core losses up to about 50% are typical. For example, theabove 302.1 permeability core had a AR/aL value of .117 before surfaceetching. This core loss was reduced to .0972 by surface etching. Theabove 323.3 permeability core had a AR/ L value of .413 unit beforeetching which was reduced to .203 unit, more than 50%, by surfaceetching. In brief, while the permeability may be decreased as much as 5%by surface etching, the core losses are reduced as much as 50%.

While 300 permeability cores with core losses within accepted standardscan be produced consistently in production utilizing the above steps,the invention also includes discovery of a novel step in the treatmentof compacted cores which further improves electrical and mechanicalproperties of molybdenum-containing Permalloy particle cores. This stepco-acts with other steps in the production of 275 and higherpermeability cores. For example, this step helps increase the mechanicalstrength of a high permeability core offsetting any weakening effect ofa plasticizer coating. This step also acts to offset the effects of thehigher pressures used in producing 275 and higher permeability cores bydecreasing core losses which could result from such higher pressures.However this novel step also improves the mechanical and electricalproperties of lower permeability cores as well.

In accordance with the invention, after annealing of the powder cores ina non-oxidizing atmosphere such as hydrogen, and after the surfaceetching, the cores are heat treated in an oxygen-containing atmosphere,for example, air. The order of these two heat treatments, that is thehydrogen annneal and the air heat treatment, cannot be reversed withoutloss of the benefits obtained by annealing in hydrogen, followed bysurface etching, followed by oxidation. It is believed that a hydrogenanneal subsequent to the heat treatment in air reduces the bonds formedduring oxidation.

The oxidation step should be carried out at a temperature between about600 F. and about 1000 F. for an interval of about 10 to 15 minutes. Apreferred oxidation treatment is applied at about 850 F. for about 15minutes. It should be understood that this oxidation treatment has atime-temperature relationship, that is, a longer period of time, forexample, 1% hours at a lower temperature, for example about 225 F., canbe utilized to provide similar oxidation, but the time involved isuneconomical and can have slightly detrimental side effects on otherproperties. In general, within the above limits, the improvement inbreakstrength is greater at higher temperatures.

Oxidation increases the breakstrength of the core as much as 75%depending on the particular core, decreases total core losses as much as25% (chiefly a decrease in eddy current losses), and markedly decreasesthe effects of moisture on a core.

Certain benefits of oxidation heat treatment are more pronounced withhigher permeability cores. A 300 permeability toroidal core, unoxidized,having a 1.06" outer diameter, about 0.5" inner diameter, and 0.44"height, breaks at 170 pounds of force per square centimeter of radialcross sectional area. An otherwise identical core, oxidized at about 850F. for 12 minutes, breaks at 260 pounds per square centimeter, anincrease in breakstrength of 50%. The breakstrength, however, of a topermeability core of similar size shows an increase in Breakstrengthmeasurements are made in accordance with the industry accepted VerticalCore Breakstrength Tes This is a mechanical test in which force isapplied on diametrically opposite sides of a painted cores outerdiameter with maximum tangential contact being made on both sides. Theramming force required to break the core is measured in pounds persquare centimeter of a radial cross section of the core.

This test provides an important parameter for designating a mechanicalcharacteristic, that is the breakstrength, of the product of the presentinvention. If the breakstrength measured in accordance with the VerticalCore Breakstrength Test is plotted versus cross-sectional areas of aradial segment of cores-in square centimeters a linear relationship isfound to exist. The minimum acceptable value of this ratio of pounds ofbreakstrength to area of a radial segment in square centimeters, for apainted core, is 290. For example, a core with an outer diameter of1.06", an inner diameter of 0.580", and a height of 0.440 has a radialsection area of 0.635 square centimeter. The minimum acceptedbreakstrength of a painted core of this size is 184.15 pounds. Thebreakstrength factor of a core having this minimum breakstrength wouldbe 184.15 divided by .635 which equals 290. The electrically insulatingpaint applied to the exterior as a final step in processing particlecores may be conventional, e.g. an enamel core paint with a thickness ofroughly 7 to 12 mils.

All molybdenum-containing Permalloy cores from 125 permeability to 300permeability show a substantial decrease in core losses after theoxidation heat treatment taught. The higher permeability cores show bestresults when annealed in the range of roughly 750' to 850 F. for about12 minutes. However with all cores of the type described, if thetemperature of the oxidation heat treatment is allowed to rise aboveabout 1000" E, the cores will deteriorate with regard to losses.

Oxidation helps solve a problem of long standing in this art, that isthe detrimental effect of humidity on permeability; see StabilityCharacteristics of Molybdenum Permalloy Powder Cores by C. D. Owens,Electrical Engineering, March 1956, pages 252-255. Past elforts havebeen concentrated on finding and applying coatings and packings forcores which would stop the detrimental effect of humidity. Theseefforts, from the point of view of practical handling problems andeconomics, have been at the limits of their capability for some time.The oxidation step taught by the invention helps to solve this problemin the core itself and, for the first time in this art, brings thehumidity problem under practical and economic control.

Humidity decreases the permeability of a core. The oxidation treatmenttaught by the invention reduces this decrease in permeability by atleast 50% in all cores and by greater amounts in the higher (300)permeability cores disclosed. For example, an unoxidized 300permeability core, with conventional electrical insulating enamelcoatings of about 7 to 12 mils total thickness on its exterior surface,shows a change of 3.3% in permeability when exposed to 95% relativehumidity in air at 150 F. for five days. Otherwise identical cores,oxidized between 575 F. and 850 F. for twelve minutes had a permeabilitychange of -1.0%. Under the same conditions 200 permeability coresunoxidized showed a -2.2% change in inductance in this test while theoxidized cores showed an average change of -1.l% in permeability.

Also the elfect of changes in temperature on permeability, i.e. thechange in permeability versus change in temperature characteristic, ismade more linear. This is partially due, it is believed, to a reductionin the effect of the difference in coefficients of expansion between thepaint on a core and the core itself, especially at lower temperatures.Evidently an oxidized core is better able to withstand the force ofcontraction of the paint on the core because of the increasedbreakstrength resulting from oxidation.

Improvement in breakstrength due to oxidation is especially beneficialwith the higher permeability cores compacted from electrically insulatedparticles having a plasticizer coating. A plasticizer dry coating weightof .06% to 0.1% ammonium lignosulfonate significantly improves corelosses but decreases the breakstrength of such cores slightly. Surfaceetching and heat treating in air cause equivalent or higherbreakstrength than that experienced with conventional cores of the samesize Without a plasticizer.

The following table lists permeability and core losses, obtained incommercial production of 300 permeability core (normal tolerance :8%permeability), for the various standard core sizes, using the teachingsof the present invention.

In describing specific embodiments of the invention, detailed steps,values, and determinations have been set forth which not only enablepractice of the invention but also provide guidelines for modificationof the specific embodiments by those skilled in the art, therefore thescope of the invention is to be determined from the following claims.

What is claimed is: 1. Particle core manufacturing process comprisingthe steps of providing finely divided magnetic particles consistingessentially of molybdenum, nickel, and iron in alloy form, selectingparticle sizes of the magnetic particles so that about 1 part by Weighthas an average particle size of about microns, about 3 parts by weighthave an average particle size of about 65 microns, and about 6 parts byweight have a particle size average not greater than about 37 microns,blending the magnetic particles with a non-agglomerating material,annealing the magnetic particles blended with nonagglomerating materialin a non-oxidizing atmosphere, electrically insulating the annealedmagnetic particles, compacting the electrically insulated particles toform a magnetic core, and annealing the compacted core in anon-oxidizing atmosphere. 2. The process of claim 1 in which theannealed magnetic metallic particles are electrically insulated with anelectrical insulation including a metallic silicate, and furtherincluding the steps of adding a plasticizer to impart flexibility to theelectrical insulation, and surface etching the annealed magnetic core tominimize skin welding effect resulting from compacting. 3. Particle coremanufacturing process comprising the steps of compacting electricallyinsulated magnetic metallic particles at a pressure in excess of tonsper square inch to form a magnetic core, then annealing the compactedparticle core in a non-oxidizing atmosphere, then surface etching theannealed core to minimize skin welding efiect resulting from compacting,and then heat treating the surface etched core in an atmospherecontaining free oxygen at a temperature above about 225 F. but nothigher than about 1000 F.

4. The process of claim 3 in which the heat treating in an atmospherecontaining free oxygen is carried out at a temperature above 600 F.

5. The process of claim 4 in which the heat treatment in an atmospherecontaining free oxygen is applied for about 10 to about 15 minutes.

6. The process of claim 3 in which the surface etching is carried outchemically.

References Cited UNITED STATES PATENTS 11/1950 Laycook 148-3155 X 2/1959 Adams et a1. 148105 6/1966 Opitz 148--104 X 3/1970 Copp 148-104 XFOREIGN PATENTS 9/1962 Japan 148-104 L. D'EWAYNE RUTLEDGE, PrimaryExaminer 10 G. K. WHITE, Assistant Examiner US. Cl. X.R.

